For almost twenty years, supercritical fluid extraction (SFE) has been a candidate for many commercial separation applications over conventional separation practices such as solvent extraction, adsorption and distillation. SFE has more favorable mass transfer rates than conventional solvent extraction because of the higher diffusion coefficients of solutes in supercritical fluids and lower viscosities of supercritical fluids. For example, in Lahiere et al., "Mass Transfer in Countercurrent Supercritical Extraction", Separation Science and Technology, 22 (2&3) (1987), 379-393, a SFE system of carbon dioxide/ethanol/water at 100 atm. and 30.degree. C. has an extraction efficiency 90% greater than that of the conventional system of toluene/acetone/water at atmospheric pressure and ambient temperature. The major factor for the enhancement in overall extraction efficiency was attributed to the differences in the dispersed phase diffusion coefficients (estimated at 2.1 (10.sup.-4) cm.sup.2 /sec for SFE and 2.6 (10.sup.-5) cm.sup.2 /sec for conventional solvent extraction). Also, the viscosity of the SFE dispersed phase was an order of magnitude less than for the conventional solvent dispersed phase of toluene/acetone/water.
However, despite these transport property advantages for the SFE system, conventional staged or continuous separation processes are still preferred for most applications. In practice few commercial SFE systems involving multi-stage countercurrent contacting have been designed and erected worldwide. This lack of commercial success is primarily because of higher capital cost associated with SFE equipment. In order to operate at pressures of about 70 to 350 bar, SFE equipment must be large enough to accommodate high supercritical fluid throughput (solvent to feed ratios greater than 20) due to inherent low solubility of solutes in the supercritical fluid. Additionally, there are high costs associated with recompressing large volumes of recycled extraction solvent. Except at very high throughput volumes, typically greater than 25 tons/day of feedstock, the cost of SFE when compared to liquid solvent extraction is higher.
A few supercritical fluid extractions utilizing countercurrent columns are in operation regardless of the economical consideration. These include separating organics from wastewater, ethanol from an aqueous stream, terpenes from citrus peel oils and fatty acid, mono-, di-, and triglycerides from fish oil and milk fat. The major impediment to further commercial exploitation of SFE countercurrent columns has been the industry perception that countercurrent column technology should be utilized for selective fractionations rather than total extractions. Selective fractionations are limited because other fractionation technologies are highly competitive. Total extractions are typically applied to aqueous feedstocks in which organic compounds are separated from a predominately water mixture as opposed to "selective/fractionation extractions" that dominate non-aqueous feedstocks. In a "total extraction" the goal is to extract all of the soluble components, whereas in a "selective extraction" the goal is to extract one or more of the easy to solubilize component(s) from one or more of the difficult to solubilize component(s). Although countercurrent column technology has been applied in a few applications for total extraction, the process economics have been somewhat disappointing due to the low solubility of solutes in supercritical carbon dioxide at the operating temperatures and pressures cited in the prior art. Thus, the majority of technical and commercial developments for SFE has been in selective/fractionation applications wherein columns operate at low pressures of between 70 and 300 bar.
To increase mass transfer flux for SFE systems making them more attractive in commercial separation processes, much research has been conducted to enhance solute solubility by utilizing co-solvents, such as methanol and ethanol, and lower critical pressure hydrocarbon solvents, such as ethane and propane. However, this has also proved to be insufficient to compete with conventional separation process because: (1) co-solvents are difficult to separate from solutes and expensive to recover and (2) hydrocarbons are flammable and explosive. Solubility enhancement by increasing temperature and pressure as another optimizing characteristic has been largely overlooked. If operating conditions existed that markedly improve the solubility of typical organic solutes, such as ethanol in carbon dioxide, then SFE might be a more cost effective industrial processing candidate displacing conventional multi-staged and continuous separation operations.
Supercritical fluids exhibit increases in solvent power for many solutes by several orders of magnitude at higher pressures and temperatures. This region of the phase diagram has been called the "enhanced solubility region". One example of this phenomenon is in the binary system of carbon dioxide/triglycerides. In the state of equilibrium, the concentration of triglycerides in the supercritical carbon dioxide increases significantly in the enhanced solubility region of pressures between 450 and 1200 bar and temperatures between 50 and 300.degree. C. This enhanced solubility of the solute in the supercritical fluid is believed to be a result of the cumulative effects from increased vapor pressure of the solute due to temperature increases and increased density of the supercritical fluid due to pressure increases.
Operation in the enhanced solubility region has heretofore been developed for solid batch extraction systems using supercritical carbon dioxide as the solvent. Applications have focused on natural product processing, such as fat extraction from oil seeds, meats and cocoa, and flavor and antioxidant extraction from herbs and spices. For example, U.S. Pat. No. 4,466,923 discloses a process for extracting a lipid from lipid containing solids, such as vegetable seed, oilseed, cereal seed germ and animal fat, at a temperature in excess of 60.degree. C. and a pressure in excess of 550 bar. U.S. Pat. No. 4,493,854 discloses a process for defatting soybean products by supercritical fluid extraction in the enhanced solubility region of at least 690 bar and 81.degree. C. Prior to extraction, the soybeans are converted to a physical state that is permeable to the carbon dioxide in a solid batch reactor. Typically, the whole bean is prepared by cracking, dehulling, and flaking. The moisture content of the prepared soybeans was noted to be of particular importance in the process and was preferably between 9 to 12 weight percent.
Further commercial exploitation of solid feedstock extraction whether using conventional pressures and temperatures or operating in the enhanced solubility region is impeded by the problems and costs associated with the conveyance of solid feedstock into and out of extraction columns. The solid feedstocks are loaded and unloaded utilizing either quick opening closures that rapidly open the lid of the extraction vessels or lock hopper systems to bring materials under pressure in one or more intermediate stages.
The quick opening closures are expensive to manufacture totaling up to 75% of the vessel cost. Lid seals of the vessels are easily damaged, resulting in high maintenance costs. Additionally, after extraction of each batch, the solvent in the vessel is lost to the atmosphere. In larger systems supercritical carbon dioxide solvent must be collected from the vessel prior to opening with an expensive carbon dioxide recovery system. The quick opening closures are awkward to operate because solids are difficult to handle in large-scale extraction systems requiring specially designed baskets or bags to be filled. In many operations manual labor represents up to 50% of the labor operating costs. To maintain a semi-continuous operation, multiple vessels piped in a complicated series manifold piping design are required. A semi-continuous process is costly, labor intensive and complex to operate. Operating in the enhanced solubility region would further add to the already exorbitant capital costs of the commercial-scale solid feedstock process.
The problems associated with lock hoppers are numerous as well. The solids must be flowable which eliminates many potential applications. Expensive valves and controls are required which can only be justified for very large extraction facilities. The valves are subject to wear and leakage and require significant maintenance. The plant layout dictates that large vessels should be utilized which are expensive to manufacture and to erect on site. Operating in the enhanced solubility range in this system would result in an unwieldy increase in the number of lock hoppers required for a nearly continuous stagewise design.
Another significant problem with solid extractions is the effect of moisture in the feed. The solids must be dried to a moisture level of 6% to 15%, prior to supercritical carbon dioxide extraction. This drying pretreatment step is an expensive process for many applications, especially fermentation broths. U.S. Pat. No. 4,495,207 discloses a process for the production of a food-grade corn germ product from a dry-milled corn germ feed stock by supercritical extraction with carbon dioxide in which the moisture content of the material should be limited to less than about 9% by weight. The '207 patent discloses that beyond this moisture level, extractability is significantly impeded.
While operation of SFE in the enhanced solubility region has been noted in the prior art as an improvement for solid batch extraction systems, almost all large commercial-scale plants operate well below 350 bar because operation in the enhanced solubility region would increase costs significantly. As discussed above, most liquid extraction columns have been designed for selective fractionations operating at lower pressures and temperatures. Liquid extraction using supercritical fluids in the enhanced solubility range has heretofore not been manufactured or developed.
Thus, a need exists in the art to provide for a supercritical fluid extraction process that will significantly lower both capital and operating costs and that will increase throughput volumes required for the economical processing of low-value/high volume commodity applications. Accordingly, it is to the provisions of such improved processes for supercritical fluid extraction that the present invention is primarily directed.